The electronics industry utilizes dielectric materials as insulating layers between circuits and components of integrated circuits (IC) and associated electronic devices. Line dimensions are being reduced in order to increase the speed and storage capability of microelectronic devices (e.g. computer chips). Microchip dimensions have undergone a significant decrease in the past decade such that line widths previously about 1 micron are being decreased to 0.18 micron with forecasts for as low as 0.10-0.05 in the next 5-10 years. As the line dimensions decrease, the requirements for preventing signal crossover (crosstalk) between chip components become much more rigorous. These requirements can be summarized by the expression RC, where R is the resistance of the conductive line and C is the capacitance of the insulating dielectric interlayer. C is inversely proportional to spacing and proportional to the dielectric constant (k) of the interlayer dielectric (ILD). Thus, shrinking the spacing requires a lower k to maintain an acceptable RC.
Historically, silica (SiO2) with a dielectric constant of 4.2-4.5 has been employed as the ILD. However, at line dimensions less than 0.18 microns, silica is no longer acceptable and an ILD with a k of 2.4-3.3 and below is needed.
Two general approaches to making a low k ILD are spin-on and chemical vapor deposition (CVD). Although both methods are capable of generating a low k ILD, CVD processes have the advantage of being able to utilize existing toolsets. Another advantage to CVD is simpler integration due to the silica-like structure of CVD-produced films compared to organic polymer films produced by some spin-on processes. CVD is also thought to have better conformality and gap filling capability than the spin-on method.
The current method of choice for dissociating or activating the reactive gases in a CVD chamber is by using a RF coupled plasma in a reaction zone above the substrate, such as that described in WO9941423. In plasma enhanced chemical vapor deposition (PECVD) the temperature required for the dissociation and deposition is typically between 100 and 400xc2x0 C., which is generally lower than temperatures required for thermal CVD.
Conventional silica (SiO2) CVD dielectric films produced from SiH4 or TEOS (Si(OCH2CH3)4, tetraethylorthosilicate) and O2 have a dielectric constant k greater than 4.0. There are several ways in which industry has attempted to produce silica-based CVD films with lower dielectric constants, the most successful being the doping of the insulating film with carbon atoms, fluorine atoms, or organic groups containing carbon and fluorine. Carbon doped silica, having the general formula, SiaObCcHd, (in which the atomic % of a+b+c+d=100%; a=10-35%, b=1-66%, c=1-35%, d=0-60%) will be referred to herein as organosilicate glass or OSG. Fluorine and carbon doped silica, having the general formula, SiaObCcHdFe (wherein the atomic % of a+b+c+d+e=100% and a=10-35%, b=1-66%, c=1-35%, d=0-60%, and e=0.1-25%) will be referred to as F-OSG. The ratio and structural arrangement of carbon, silicon, oxygen, fluorine, and hydrogen atoms in the final ILD is dependent on the precursors chosen, the oxidant, and the CVD process conditions, such as RF power, gas flow, residence time, and temperature.
Doping the silica with carbon atoms or organic groups lowers the k of the resulting dielectric film for several reasons. Organic groups, such as methyl, are hydrophobic; thus, adding methyl or other organic groups to the composition can act to protect the resulting CVD deposited film from contamination with moisture. The incorporation of organic groups such as methyl or phenyl can also serve to xe2x80x9copen upxe2x80x9d the structure of the silica, possibly leading to lower density through space-filling with bulky CHx bonds. Organic groups are also useful because some functionalities can be incorporated into the OSG, and then later xe2x80x9cburned outxe2x80x9d or oxidized to produce a more porous material which will inherently have a lower k. The incorporation of voids or pores in a low dielectric constant material will result in reductions in the dielectric constant proportional to the amount of porosity. While this is beneficial, the amount of porosity incorporated into the film must be balanced with the deleterious effects that the introduction of pores will have on the mechanical properties of the film. Thus the optimum amount of porosity will be material dependant.
Doping the ILD with fluorine provides low polarizability, thus leading to lower k. Fluorine-containing organic groups such as CF3 are very hydrophobic, so their presence will also serve to protect the silica from contamination with moisture.
While fluorinated silica materials have the requisite thermal and mechanical stability to withstand very high temperatures (up to 500xc2x0 C.), the materials"" properties (e.g., low water sorption, mechanical properties) are susceptible to being compromised when large amounts of fluorine are incorporated into the material. Fluorinated organic materials, such as poly(tetrafluoroethylene) despite having very low k values down to 2.0 or less, have not shown sufficient stability to the temperatures experienced during subsequent processing steps involved in the manufacture of an integrated circuit. Organic polymers in general do not possess sufficient mechanical strength for processing under current conditions. Also, fluorocarbon polymers can have other drawbacks such as poor adhesion, potential reaction with metals at high temperature, and poor rigidity at high temperature in some cases.
One way to incorporate carbon into an ILD is by using an organosilane such as methylsilanes (CH3)xSiH4-x as a silicon source in the PECVD reaction. WO9941423 and U.S. Pat. No. 6,054,379 describe the reaction of a silicon compound containing methyl groups and Sixe2x80x94H bonds with nitrous oxide (N2O) oxidant to give an SiOC film with a carbon content of 1-50% by atomic weight and a low dielectric constant.
U.S. Pat. No. 6,159,871 disclose methylsilanes (CH3)xSiH4-x (x is 1-4) as suitable CVD organosilane precursors to OSG low k films. Materials with 10-33% carbon content, by weight, and k less than 3.6 are reported.
An article by M. J. Loboda, et al., entitled xe2x80x9cDeposition of Low-K Dielectric films using Trimethylsilane,xe2x80x9d in Electrochemical Soc. Proc., Vol. 98-6, pages 145 to 152, describes the use of trimethylsilane in a PECVD process to provide films with a k of 2.6 to 3.0.
Other patents describe the use of phenyl or vinyl containing organosilane precursors in producing dielectric films. For example, U.S. Pat. No. 5,989,998 discloses the preparation of PECVD low k films from, for example, (C6H5)xSiH4-x or (CH2xe2x95x90CH)xSiH4-x) (x is 1,2 or 3), and an oxidizing gas. WO 9938202 discloses dielectric films deposited from phenyl or methylsilanes with hydrogen peroxide as the oxidant and the addition of oxygen to aid in the association between the silicon compound and the oxidant.
WO 9941423 and EP 0935283 A2 disclose siloxanes such as H(CH3)2SiOSi(CH3)2H, (CH3)3SiOSi(CH3)3, and cyclic (xe2x80x94OSiH(CH3)xe2x80x94)4 as precursors for PECVD produced OSG films.
Silyl ethers (alkoxysilanes) have also been disclosed as precursors for dielectric films. EP 0935283 A2 discloses methoxy and ethoxysilanes, such as (CH3)2Si(OCH3)2 and (CH3)(C6H5)Si(OCH3)2. U.S. Pat. No. 6,086,952 discloses a method of forming thin polymer layers by blending reactive p-xylylene monomers with one or more comonomers having silicon-oxygen bonds and at least two pendent carbon-carbon double bonds, such as tetraallyloxysilane.
U.S. Pat. No. 6,171,945 discloses a process in which organosilanes with xe2x80x9clabilexe2x80x9d ligands, such as formyl or glyoxyl groups, are reacted with a peroxide compound at the surface of the substrate, and are subsequently removed by annealing to provide a porous ILD.
U.S. Pat. No. 6,054,206 discloses deposition of a film using an organosilane and an oxidant, followed by removal of the organic component in the film with an O2 plasma to generate a porous silica material.
F-OSG is typically prepared by CVD using organosilane precursors with methyl or phenyl Cxe2x80x94F bonds. For example, WO 9941423 discloses the use of PECVD for a number of organosilane precursors with sp3-hybridized Cxe2x80x94F bonds such as (CF3)SiH3.
JP11-111712 describes the preparation of films from the deposition of (CF3)Si(CH3)3, followed by thermal treatment with O2 to generate an insulating film with a k of 2.5 to 2.6.
U.S. Pat. No. 6,020,458 teaches that the use of sp2-hybridized Cxe2x80x94F bonds, such as those in (C6F5)SiH3, is preferable due to the stronger Cxe2x80x94F bond strength, leading to greater thermal stability of the resultant ILD films.
JP10-088352 discloses the possible use of (R1O)nSi(OR2)4-n (R1 is a fluorinated alkyl chain and R2 is an non-fluorinated alkyl chain) as a precursor for a fluorine containing silicon oxide film. U.S. Pat. No. 5,948,928 discloses the possible use of fluoroacetate substituted silanes as insulating film precursors. However, both JP 10-088352 and U.S. Pat. No. 5,948,928 are directed to the preparation of fluorosilicate glass (FSG), not F-OSG films.
Inadequacies exist in currently known CVD precursors and the corresponding ILD films. One problem is that it can be difficult to include all atoms or functionalities desired in the film in the same precursor molecule, in the desired ratio to produce a SiaObCcHd or SiaObCcHdFe film, in which the total atomic % of a+b+c+d+e=100%, and a=10-35%, b=1-66%, c=1-35%, d=0-60%, and e=0-25%. Heterogenous mixtures of precursors can be used, but this is less desirable to a single-source precursor for process reasons.
The deposited ILD films should be able to withstand temperatures up to 450xc2x0 C., since some Sixe2x80x94H, Sixe2x80x94C, Cxe2x80x94H, or Cxe2x80x94F bonds can be broken at high temperatures. Release of F ions or radicals can etch the film or other components.
The low k ILD must also have suitable mechanical strength. When new low k films are substituted for traditional SiO2, subsequent integration process steps are impacted. The deposited films need to stand up to subsequent processes including chemical mechanical planarization (CMP), cap and barrier layers, and photoresist adhesion, stripping, etching and ashing.
Studies have indicated that current OSG candidates produced from trimethylsilane (3 MS) or tetramethylsilane (4 MS) limit the dielectric constant to the range of 2.6-2.9 with modulus/hardness values in the range of 4-11/0.25-1.4 GPa (Lee et al., 198th Meeting of The Electrochemical Society, October 2000, Section H-1, Abstract No. 531.)
Precursors which are safe to handle and possess long shelf lives (greater than 1 year) are also desirable. Silane (SiH4) is a pyrophoric gas, and methyl, dimethyl, or trimethylsilane are all highly flammable gases.
Finally, the CVD organosilicon precursors need to be readily available and affordable.
Despite the foregoing developments, there have not been any examples in the prior art that successfully combine the desired mechanical and electrical properties that are paramount for integrating low k dielectric materials in integrated circuits.
The need therefore remains for precursors of low k ILD films that solve the problems associated with known precursors, such as those described above.
This invention is directed to a method of making ILD films having a k of 3.5 or less, preferably 3 or less, by chemical vapor deposition (CVD), such as plasma enhanced CVD (PECVD) or thermal CVD, using specific organosilicon precursors. This invention is also directed to the films produced therefrom, and to methods of using the films.
The low k ILD films can be deposited as either OSG (SiaObCcHd) or F-OSG (SiaObCcHdFe) films (wherein the atomic % of a+b+c+d+e=100% and a=10-35%, b=1-66%, c=1-35%, d=0-60%, and e=0-25%). An oxidant such as N2O, O2, O3, or H2O2 may be used in the CVD reactor, but may not be required in all cases since many of the precursors already incorporate Sixe2x80x94O bonds. Novel porous ILD films may also be produced using these specific organosilicon precursors.
The specific organosilicon precursors of this invention are silyl ethers, silyl ether oligomers, and organosilicon compounds containing reactive groups.
The silyl ethers of this invention can have a structure as shown in structures I-VII, below, where x is an integer of 1 to 3, y is 1 or 2, and z is an integer of 2 to 6. 
R1 can be one or more of H, fluorine, a branched or straight chain C1 to C6 alkyl, a C3 to C8 substituted or unsubstituted cycloalkyl, a C6 to C12 substituted or unsubstituted aromatic, a partially or fully fluorinated C1 to C6 alkyl, a partially or fully fluorinated C3 to C8 cycloalkyl, or a partially or fully fluorinated C6 to C12 aromatic. Examples of R1 are unfluorinated, partially fluorinated, or fully fluorinated methyl, ethyl, propyl, isopropyl, cyclopentyl, cyclohexyl, methylcyclohexyl, phenyl, and tolyl.
R2 can be one or more of a C6 to C12 substituted or unsubstituted aromatic such as phenyl, tolyl, or pentamethylphenyl, a fluorinated straight chain, branched chain or cycloalkyl up to C8F15, such as trifluoromethyl and pentafluoroethyl, or partially or fully fluorinated aromatic group, such as C6H3F2 or C6F5.
R3 can be one or more of R2, a C1 to 6 linear or branched alkyl, or a C3 to C8 substituted or unsubstituted cycloalkyl, such as methyl, cyclohexyl, phenyl, or tert-butyl.
R4 can be one or more of a C1 to C6 linear or branched alkyl, C3 to C8 substituted or unsubstituted cycloalkyl, or C6 to C12 substituted or unsubstituted aromatic and can be partially or fully fluorinated. Examples of R4 are unfluorinated, partially fluorinated, or fully fluorinated methylene, ethylene, and phenylene.
Examples of compounds having structure VII are disilane or trisilane containing alkoxy ligands, such as H(CH3O)(CH3)Sixe2x80x94Si(CH3)(OCH3)H.
Structures I and II contain monodentate alkoxy groups, with one Sixe2x80x94O bond per OR2 or OR3 group. In structure II, the R4 alkoxy group can be bidentate with one Sixe2x80x94O bond and one Sixe2x80x94C bond per R4 group. In structure IV, R4 is a bidentate alkoxy structure with two Sixe2x80x94O bonds per R4 group. In structures V and VI, the R4 alkoxy group forms bridged structures with two Sixe2x80x94O bonds.
A major advantage in using silyl ethers of this invention, having Sixe2x80x94Oxe2x80x94C bonds, to dope organic or organofluorine groups into the ILD film, instead of silanes having only Sixe2x80x94C and Sixe2x80x94H bonds, is that Sixe2x80x94Oxe2x80x94C bonds are more convenient to form than Sixe2x80x94C bonds. In addition, starting materials for silyl ethers are readily available and inexpensive, and silyl ethers are safer to handle than silanes. Most of the silyl ethers described in this invention are flammable liquids, while silane (SiH4) is a pyrophoric gas, and methyl-, dimethyl-, and trimethyl-silane are highly flammable gases.
The organosilicon precursors containing reactive groups have the general structure R14-xSiR5x in which x is an integer of 1 to 3, R1 is as described above for structures I-VII, and R5 is a reactive group. A reactive group is typically defined as chemical bond(s) between two (or more) atoms which can be broken using a minimal amount of energy, is strained, or is not in a thermodynamically preferred configuration, and has a propensity to form new chemical bonds or crosslinked structures with other chemical species. Reactive groups can aid in the crosslinking of the deposited film which enhances the thermal stability and mechanical strength of the ILD film. Examples of reactive side groups include C2 to C10 epoxides such as ethylene oxide or 2-ethyloxirane, C2 to C8 carboxylates such as methyl acetate or ethyl acetate, C2 to C8 alkynes such as propyne, ethyne, and phenylethyne, C4 to C8 dienes such as 1,3-butadiene, 1,4 octadiene or 1,3 cyclopentadiene, C3 to C5 strained rings such as cyclopropane or 2-cyclobutene, and C4 to C10 organic groups, such as tert-butyl, tert-butyloxide, or adamantane, that can sterically hinder or strain the organosilicon precursor. Examples of organosilicon compounds containing reactive groups are trimethylsilylacetylene, 1-(trimethylsilyl)-1,3-butadiene, trimethylsilylcyclopentadiene, trimethylsilylacetate, and di-tert-butyoxydiacetoxysilane.